﻿155 Geology and Natural History. 



either in the number and form of the jaws {Diopatra, etc.), or in 

 the ceplialic appendages, as in SerpulidcB, wliere one of the 

 branchia?, on one side, is often transformed into an operciihim ; or 

 in various other organs, in diti'erent groups ; while the dorsal and 

 ventral regions are often quite as strongly contrasted as in any 

 mollusk ; and in some cases the body is compressed laterally 

 {Ammotrypane). But on the other hand, mollusks are often 

 nearly or quite symmetrical (many Nudibranches, Chitons, Ptero- 

 pods, Ce])halopods) and are frequently cylindrical or even de- 

 pressed {Doris, Chitons, Cephalopods, Pteropods). In fact form 

 is a very poor character for characterizing any large group of 

 animals, and should have little or no weight in this case. The 

 ventral connections of the " locomotor muscles'" in mollusks and 

 their lateral and dorsal attachment to the integument in worms 

 are given as distinctive. But we generally find the locomotor 

 muscles of animals connected with the locomotive organs, wher- 

 ever these may be situated. So in Pecten we find that the main 

 loconu)tor muscles are attached laterally to the shell, that being 

 its principal organ of locomotion ; and in Cephalopods we find 

 them on the sides and back, as well as ventrally, so that the man- 

 tle may be used as a locomotive organ. On the other hand, many 

 worms, like Aphrodite, Lepidanotits, many leeches, Trematodes, 

 and other worms, both liigh and low, have their locomotive organs 

 as truly ventral as those of Gastx'opods. "In the Mollusk the 

 tegumentary envelope is prolonged, and oftentimes continuous, 

 forming a sac or mantle, inclosing a conspicuous cavity, and pro- 

 tecting the gills." This is, indeed, a valuable character, but not 

 accurately stated, for the mantle does not always form a "cavity," 

 and is often nearly or quite abortive, and the gills are often situ- 

 ated on the back or sides, as in the Nudibranchs. But in this 

 character the Brachiopods agree with the Mollusks, and not with 

 the worms. " In the worm the digestive canal is straight, rarely 

 convoluted, and suspended freely in the perivisceral cavity." " In 

 the Mollusk, the intestine is always convoluted, not suspended 

 freely in the perivisceral cavity, but intimately blended, or united 

 with other organs." The intestine varies immensely in both 

 groups, according to the food of the species, and cannot be prop- 

 erly used as a character for separating two sub-kingdoms. Among 

 Sipunculoid Worms (as stated on p. 26) the intestine is generally 

 very long and greatly convoluted, and may terminate either ante- 

 riorly or posteriorly. 



In most Xemerteans, Planarians, and Trematode worms the 

 intestine is not " freely suspended," but firmly united to the other 

 organs and the tegumentary system. " In Vermes there is a 

 peculiar depuratory apparatus characteristic of all. In the Anim- 

 lata this apparatus takes the shape of bilaterally symmetrical 

 tubes, in pairs, opening externally and communicating with the 

 perivisceral caviity, by distinct independent infundibuliform ori- 

 fices. In the MoUusca, with the exception of certain Cephalo- 

 poda, nothing of the kind is found, and where such communi- 



